Polybutylene terephthalate (PBT) resin is widely used as a conventional engineering plastic. Although PBT resin is usually employed to form injection-molded articles, it is also sometimes extrusion-molded to form films, for example.
It is well known that resin films may be produced by either a T-die method or an inflation molding method. In this regard, it was typically thought to be difficult to produce polybutylene terephthalate resin films by an inflation molding method. It has recently been proposed to form films having excellent properties which can be produced by inflation-molding processes. In this regard, please see Japanese Patent Laid-Open No. 63-251,219 and U.S. Pat. No. 4,869,864 (incorporated hereinto by reference).
Films are used in a number of end-use applications. For example, highly transparent films, in particular, transparent polybutylene terephthalate resin films, having excellent thermal and chemical resistance are needed in the packaging field, especially for food packaging. However, highly transparent films can usually not be obtained under any molding conditions by means of conventional upward inflation molding methods using air-cooling apparatus. That is, transparent films cannot be obtained using ordinary air-cooling upward inflation molding techniques since the films tend to cloud due to crystallization of PBT resin--i.e., because polybutylene terephthalate resin has a high crystallization rate.
The present invention is directed to films that are highly transparent and exhibit excellent thermal and chemical resistance properties that can be produced via inflation-molding conditions using a resin composition which includes a polybutylene terephthalate resin component having a particular intrinsic viscosity, and a polycarbonate resin component having a particular intrinsic viscosity incorporated into the composition in a particular amount.
The term "transparent" as used herein and in the accompanying claims is intended to refer to a haze value (cloudiness value) of the film of less than 10% as determined according to ASTM D 1003.
More specifically, the films of this invention will be formed by an upward inflation molding technique using a resin composition which is comprised of 100 parts by weight of a polybutylene terephthalate resin having an intrinsic viscosity IV.sub.a (as determined in o-chlorophenol at 25.degree. C.) of at least 1.0, and between 15 to 45 parts by weight of a polycarbonate resin having an intrinsic viscosity IV.sub.b (determined in methylene chloride at 20.degree. C.) of at least 0.4. The polybutylene terephthalate resin component and the polycarbonate resin component are present such that the ratio .eta..sub.A /.eta..sub.B is between 0.35 to 1.5, wherein .eta..sub.A is the melt viscosity of the polybutylene terephthalate resin at a shear rate of 100sec.sup.-1 at 260.degree. C., and .eta..sub.B is the melt viscosity of the polycarbonate resin at a shear rate of 100sec.sup.-1 at 260.degree. C.
The resin composition is most preferably extruded at an extrusion temperature T(.degree. C.) during the inflation molding step so as to satisfy the equation (1): EQU 240.degree. C..ltoreq.T(.degree.C.).ltoreq.(MP-26)+(53.times.IV.sub.a)
where MP is the melting point of the resin composition as determined by differential thermal analysis at a rate of 10.degree. C. per minute. The inflation molding step itself generally includes extruding the resin composition continually through an annular die in an upward direction so as to form a tubular film of the extruded resin, and introducing cooling air into said tubular film so as to achieve an inflation ratio D.sub.1 /D.sub.2 of between 1.0 to 3.0, where D.sub.1 is the diameter of the tubular film exiting the extrusion die, and D.sub.2 is the maximum diameter of the tubular downstream of the extrusion die. The thickness of the film is advantageously 50 .mu.m or less.
Further aspects of the present invention will become more clear after careful consideration is given to the following detailed description of the preferred embodiments thereon.